1. Field of the Invention
The present invention relates to an inverter for adjusting an output voltage according to a current value of a drive current supplied to rotation drive means (to be referred to as a motor herebelow) of a motor or the like, and in particular, to an inverter in which the torque characteristic of a motor is remarkably improved when the motor is supplied with a power in a low-frequency range.
2. Description of the Prior Art
In general, most of various apparatuses utilized in various industrial fields are driven by rotation drive means such as a motor. For the most effective control of such apparatuses using the rotation drive means, the rotary frequency of the rotary drive means is controlled, which is effected, for example, by the following concrete methods: .circle. Pole change, .circle. Primary voltage control, .circle. Primary frequency control, and .circle. Eddy current coupling control. Among these methods, according to the primary frequency control method, the frequency of the power supplied to the motor or the like is controlled, thereby controlling the rotary frequency. When the primary frequency f is changed and if the voltage V applied to the motor is varied in proportion to the frequency f, the output from the motor develops a substantially constant torque characteristic.
FIG. 1 shows a characteristic diagram of the characteristic of the output supplied from the conventional inverter to a motor which is the load and is used as a rotation drive means. In this diagram, the output voltage V.sub.0 and the output frequency f.sub.0 are plotted along the ordinate and abscissa, respectively in which the voltage/frequency pattern of the inverter output is represented by the basic pattern P.sub.1 and the adjust pattern P.sub.2. The adjust pattern P.sub.2 is adjusted according to the load current, namely, an input current to the motor. If the load current exceeds the preset current value, the control effects an increase in the output voltage in the entire range of the output frequencies, thereby automatically achieving a torque boost of the motor. By use of the automatic torque boost, a higher load is also acceptable for the same inverter capacity, namely, the same rated output current. Consequently, a motor having the auto torque boost function is suitably applied, for example, to apparatuses such as a mixer and a transport apparatus in which a high load is required at the start thereof and the load is lowered during the operation or to apparatuses such as a conveyor in which the load is intermittently increased during the stationary operation and is stabilized after the unstable condition is finished.
However, when using the conventional inverter even with such an auto torque boost function, the influence, for example, of a voltage drop caused by a resistance of the coil of the motor in the low-frequency range prevents the output current from being smoothly supplied to the motor side, causing the torque generated for the low frequency to become lower than the rated torque as shown by a curve a in FIG. 2, which leads to a problem that the torque boost characteristic cannot be developed in the low-frequency range even when the load on the motor is great, because the input current is small.
In addition, when the motor is in the acceleration process, the slippage is increased in the low-frequency range due to an insufficient torque, which leads to a problem that the response of the motor or the like to a speed command is deteriorated.
Moreover, when the load torque is great in the low-frequency range, the rotation speed of the motor is lowered, and when the inverter output is increased up to the frequency range in which the excess current as shown by a curve b or c in FIG. 2 flows to the motor, the current flowing through the motor exceeds the output current permissible for the inverter, which leads to a problem that the motor is stopped.